The Essay concerning Human Understanding was published at the end of 1689.1 It sold well, and within three years Locke was planning revisions for a second edition. Among those whose “advice and assistance” he sought was the Irish scientist William Molyneux. Locke had begun a correspondence with Molyneux a few months before, after the latter had lavishly praised the Essay and its author in the Epistle Dedicatory of his own Dioptrica Nova, published early in 1692. Here was a man, Locke concluded, whose judgment one could trust. He returned Molyneux’s compliment in the Essay’s new edition, calling him “that very Ingenious and Studious promoter of real Knowledge, ... whom I am proud to call my friend”